As digital data services proliferate, the need for data channels to carry these services into homes and businesses likewise increases. It has become common to install special wideband transmission facilities in those places where such wideband digital services are desired. These special transmission facilities are expensive, require continuous maintenance, often in the outside plant portion of the facility, and require expensive terminal equipment. It would be of considerable economic benefit if the twisted-pair telephone wires currently extending to virtually all of the homes and businesses in the country were able to carry such wideband digital services.
One of the most critical functions in a digital receiving system is symbol synchronization. In such a system, the received signal must be sampled at the baud rate in order to detect the transmitted pulse signal. Moreover, the choice of timing phase for operating the sampling device is crucial in minimizing errors due to noise and intersymbol interference. Choosing the correct sampling phase is even more critical in transmission systems using narrow band twisted-pair telephone lines to transmit the wideband digital signals necessary to support new services. In such systems, intersymbol interference is especially great and hence deriving a correct timing signal is especially difficult. The data signal in such a system is so distorted by intersymbol interference that normal methods of timing recovery operate marginally or not at all.
The prior art has typically used analog signal processing of the incoming data signal to derive a timing signal. Most digital receivers, however, rely on digital processing to recover the digital information modulated on the incoming pulse train. That is, the received signal is sampled at discrete time intervals and converted to digital amplitude magnitudes. All further processing is carried out using digital circuitry, typically realized with very large scale integration (VLSI). Since circuit cost and complexity increases with the sampling rate, it has become common to sample the incoming signal at the lowest possible rate, i.e., the baud rate. Unfortunately, baud rate sampling does not permit reconstruction of the analog signal waveform due to aliasing distortion and hence analog timing recovery cannot generally be used in digital receivers operating at the baud rate.
One prior art technique for overcoming this problem is taught by K. H. Mueller and M. Muller in an article entitled "Timing Recovery in Digital Synchronous Data Receivers," IEEE Transactions on Communications, Volume COM-20, May 1976, pages 516-530. In the Mueller et al. article, a preselected timing function is used to describe the optimal sampling instant. The coefficient values of this timing function are then estimated from the arriving signal samples. Since timing jitter depends on the actual pulse sequence transmitted as well as the impulse response, the timing function estimates have a relatively high variance. A similar type of timing recovery system is disclosed in "Data-Sequence Selective Timing Recovery for PAM Systems," by A. Jennings and B. R. Clarke, IEEE Transactions on Communications, Volume COM-33, July 1985, pages 729-731. Unfortunately, the Jennings et al. timing recovery system relies on a predetermined, but randomly occurring, pulse sequence for adjusting the timing phase. The problem of stably recovering timing information from an incoming digital signal sampled at the baud rate therefore remains.